In general, yes I do think that we should get rid of such informal rules. And I would appreciate a law that e.g. ensures an employer can not discriminate against men wearing dresses or skirts. For what it's worth, there have been protest by bus drivers, who are not allowed to wear shorts in the summer, who showed up in skirts on a hot day.

If we change the garment from abayas to pants it would be "to ban male students from wearing pants in school", meaning they'd be forced to wear skirts or dresses. But two points make this different from the OP:

1. Since this is not linked to religion it has a slightly different spin. I can't put it into words that well, but a guy choosing to wear a skirt is just that, a clothing choice. But Islam is pretty explicit that women should cover themselves. So if a guy goes against the informal law people would make fun of him. If an Islam woman wears short clothes she is not only made fun of, but can also get in trouble with her entire community.
2. While dresses/skirts are almost exclusively worn by women, pants are worn by men and women. So a guy wearing pants is not the outlier, he is wearing the gender neutral clothing. If abayas are also worn by a significant fraction of male students in France I would heavily oppose the proposed ban, but I found nothing that would indicate such a practice.

And for some reason it’s always only about Muslim women! Other religions which can also coerce or force family members to follow a certain dress code, not a single word about them.

Because right now and in Europe those are rather rare. Afaik there's some Christian groups where women always wear long skirts, but those groups tend to get called "cults" ("Sekten" here in Germany) so I don't really see a double standard there.

That said, there are surveys regarding why women wear hijabs and - at least in Germany - those say that the vast majority wears them voluntarily.